The present invention relates to a satellite-signal reception-processing apparatus included in a GNSS (Global Navigation Satellites System) such as a GPS (Global Positioning System) as an apparatus for receiving signal electric waves from a plurality of artificial satellites and finding the position and velocity of the apparatus, and relates to a satellite-signal reception-processing method adopted in the apparatus.
In a GNSS such as a GPS for measuring the position of a moving body by making use of artificial satellites each referred to hereafter simply as a satellite, a GSP receiver serving as a satellite-signal reception-processing apparatus has a function for receiving signals from four or more satellites and computing the position and velocity the receiver itself from the received signals. In addition, the GSP receiver also has a function for notifying the user of the computed position and velocity.
In the GPS system, a signal received from a satellite is a signal spread into a spectrum by a spreading code referred to as a C/A (Clear and Acquisition) code. The C/A code is a code pertaining to a PN (pseudo random noise) system with a transmission signal velocity (a chip rate) of 1.023 MHz and a code length of 1023. The C/A code is a spreading code typically composed of a gold code. The code of the PN system to which the C/A code pertains varies from satellite to satellite.
A signal received from a satellite is a signal obtained as a result of a BPSK (Binary Phase Shift Keying) modulation process carried out on a carrier having a frequency of 1575.42 MHz by using a signal spread into a spectrum by using a spreading code. In the following description, a signal received from a satellite is also referred to as a satellite signal.
As described in Japanese Patent Laid-open No. 2003-258769, a GPS receiver receives such a satellite signal from a satellite, demodulates the received satellite signal to obtain a navigation message including an almanac (information on times) and an ephemeris (information on the locus of a satellite). The GPS then stores the navigation message including these pieces of information in a memory.
Then, the GPS receiver finds the three-dimensional position of its own from the information on times, the locus of each satellite and a delay time incurred by each satellite signal by solving simultaneous equations. The delay time incurred by a satellite signal is defined as a difference between the time of an arrival of the satellite signal at the GPS receiver and the time of a transmission of the satellite signal from the satellite. To put it in detail, the GPS receiver computes the positions of four or more satellites and pseudo distances to the satellites from the loci of the satellites, times of transmissions of spreading codes for signals received from the satellites and times of arrivals of the spreading codes. In this case, the position of any specific one of the satellites is a position at which the specific satellite is located at the time of a transmission of a spreading code for a signal received from the specific satellite. On the other hand, the pseudo distance to any specific one of the satellites is a distance to a position at which the specific satellite is located at the time of a transmission of a spreading code for a signal received from the specific satellite. The GPS receiver then establishes the simultaneous equations involving the three-dimensional coordinates of the position of the GPS receiver as well as four or more time errors between the GPS receiver and satellites as unknown variables to be found from the equations.
Signals received from four or more satellites are required for the purpose of positioning because an error in internal time between the GPS receiver and each of the satellites exists and it is necessary to get rid of effects of the error. It is to be noted that, in this specification, the positioning includes not only a computation of the position of the GPS receiver, but also a calculation of the velocity of the receiver.
Each equation of the simultaneous equations is a quadratic equation not including a term of a product of different unknown variables. In general, by giving a proper initial value close to each solution to the simultaneous equations for each solution and applying an iteration method such Newton's method, the equations can be solved. In accordance with Newton's method, a given equation is subjected to locally linear approximation at an initial value close to a solution to the equation. The linear simultaneous equations obtained as a result of the approximation using the initial values are solved first to produce tentative solutions. Then, the tentative solutions are used as next initial values. This operation is carried out iteratively. As differences between the present solutions and the immediately preceding solutions are each converged to a value smaller than a predetermined error, the present solutions are taken as final solutions to the simultaneous equations.
In addition, the GPS receiver finds not only the three-dimensional position of its own and a time, but also the three-dimensional velocity of its own by computing a Doppler frequency of the carrier transmitted by each of the four or more satellite signals. A Doppler effect is also observed in a C/A code. In order to satisfy desired precision of the three-dimensional velocity being found, however, the Doppler frequency is generally used.
To put it in detail, the GPS receiver computes the positions of four or more satellites and velocities of the satellites from the loci of the satellites and times of transmissions of spreading codes for signals received from the satellites. In addition, by using a position found as described earlier as the position of the GPS receiver and using the frequency of a carrier transmitted by each of the satellites, the GPS receiver then establishes simultaneous equations involving the three-dimensional velocity of the GPS receiver and intermediate-frequency carrier errors as unknown variables to be found from the equations. In this case, the frequency of a carrier transmitted by each of the satellites is a frequency obtained from a synchronization sustenance section as a frequency in a stable state.
Normally, in a process carried out by the GPS receiver to compute its own position and velocity, first of all, visible satellites are picked out. If no loci have been stored in the GPS receiver, electric waves received from all satellites are searched for receivable ones. The receivable electric waves are each treated as a wave received from a visible satellite. If loci have been stored in the GPS receiver, on the other hand, any satellites existing above the horizon in accordance with calculation based on the loci are each picked out as a visible satellite.
Then, for a signal received from each visible satellite, the GPS receiver carries out processing to sustain synchronizations of the spreading code and the carrier wave. Subsequently, the GPS receiver computes its own position and velocity by using signals received from satellites for which the synchronizations can be sustained. However, not all signals received from all satellites for which the synchronizations can be sustained are used to compute the position and velocity of the GPS receiver. That is to say, only signals received from specific satellites for which the synchronizations can be sustained are used. The specific satellites are satellites, the signals received from which can be used to compute the position and velocity of the GPS receiver with at least a predetermined degree of precision.
The GPS receiver in related art identifies a set including a plurality of satellites each satisfying all three conditions listed below as a set of satellites usable for computing the position and velocity of the GPS receiver. The three conditions are listed as follows:    (a) its locus is already known;    (b) the time of a transmission of its spreading code is already known; and    (c) the frequency of its carrier is already known as a stable frequency.
Next, a combination of four or more satellites is picked out from the set of satellites usable for computing the position and velocity of the GPS receiver. The satellites picked out from the set are satellites presumed to be those from which the position and velocity of the GPS receiver can be computed with a degree of precision exceeding a predetermined threshold value. In general, for the process to pick out a combination of four or more satellites from the set of satellites usable for computing the position and velocity of the GPS receiver, as a precision indicator showing the effect of the location of a satellite on the precision, a DOP (Dilution Of Precision) value is used.
That is to say, the GPS receiver finds a DOP value for a combination of four or more satellites to be used for computing the position and velocity of the GPS receiver. If the found DOP value is found equal to or smaller than a threshold value determined in advance, the GPS receiver computes the position and velocity of its own. If the found DOP value is found greater than the threshold value determined in advance so that the precisions of the position and the velocity would deteriorate, on the other hand, the GPS receiver cancels the computations of the position and the velocity. In this case, the threshold value of the DOP value is common to the computations of the position and the velocity.
It is to be noted that, in either the computation of the position or the computation of the velocity, if solutions to simultaneous quations for five or more satellites can be found, as is generally known, solutions to linear simultaneous equations are found as solutions giving a minimum sum of squared errors. That is to say, in general, the greater the number of satellites to be used for computing the position and velocity of the GPS receiver, the higher the degrees of precision.